A semiconductor processing apparatus for the deposition of thin films on semiconductor substrates, such as a CVD-apparatus, a term which in this text may be construed to comprise ALD-apparatus, may typically include a reactor and a gas supply system for the supply of process gases, e.g. precursors, to that reactor. The gas supply system may be complex and expensive, in particular when the process materials/precursors are solid or liquid at about room temperature and thus require melting and/or evaporation, or sublimation before being introduced into the reactor. In case the semiconductor processing apparatus includes a plurality of reactors, the gas supply system may be at least partially shared between the plurality of reactors. Accordingly, savings in both manufacturing and maintenance costs, and in space may be achieved.
When a shared gas supply system is used for a plurality of reactors, the process gases may be supplied to all reactors simultaneously. However, the gas flows to each individual reactor may not be very well controlled. Providing flow control devices for each individual reactor may render the system unduly complicated and expensive. Alternatively, the process gases may be supplied to the plurality of reactors in a sequential way, one reactor at a time. In this way all components of the shared gas source, including eventual flow control components can be shared among the plurality of reactors. In particular for processes using process gas pulses this might be a very economic option. In between gas pulses for one reactor, when the gas supply system is idle or the gas flow is diverted to a vent line or an exhaust, the shared gas supply system may supply gas pulses to another system(s) without (much) loss of process time. However, a common problem with shared gas supply systems is that the plurality of valves, gas line nodes and gas line sections in such a system, required to selectively supply one or more process gases to the various reactors, may retain a certain volume of stagnant fluid. This volume may be referred to as dead volume. A gas supply system inflicted with dead volume may resist effective purging, which may, for instance, cause one reactor to be unintentionally supplied with process gas meant to be supplied to another reactor, or undesired mixing of mutually reactive process gases within the gas lines of the system.